Field of the Invention
This invention relates to methods for shutting down and storing a fuel cell system at below freezing temperatures. In particular, it relates to methods for shutting down and storing an automotive fuel cell system comprising a solid polymer electrolyte fuel cell stack.
Description of the Related Art
Fuel cells such as solid polymer electrolyte fuel cells electrochemically convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. Solid polymer electrolyte fuel cells generally employ a proton conducting, solid polymer membrane electrolyte between cathode and anode electrodes. A structure comprising a solid polymer membrane electrolyte sandwiched between these two electrodes is known as a membrane electrode assembly (MEA). In a typical fuel cell, flow field plates comprising numerous fluid distribution channels for the reactants are provided on either side of a MEA to distribute fuel and oxidant to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell. Water is the primary by-product in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is of order of 1V, a plurality of cells is usually stacked together in series for commercial applications in order to provide a higher output voltage. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.
Along with water, heat is a significant by-product from the electrochemical reactions taking place within a fuel cell. Means for cooling a fuel cell stack is thus generally required. Stacks designed to achieve high power density (e.g. automotive stacks) typically circulate liquid coolant throughout the stack in order to remove heat quickly and efficiently. To accomplish this, coolant flow fields comprising numerous coolant channels are also typically incorporated in the flow field plates of the cells in the stacks. The coolant flow fields may be formed on the electrochemically inactive surfaces of the flow field plates and thus can distribute coolant evenly throughout the cells while keeping the coolant reliably separated from the reactants.
In certain applications, fuel cell stacks may be subjected to repeated on-off duty cycles involving storage for varied lengths of time and at varied temperatures. It is generally desirable to be able to reliably start-up such stacks in a short period of time. Certain applications, like automotive, can require relatively rapid, reliable start-up from storage conditions well below freezing. This has posed a significant challenge both because of the relatively low rate capability of cells at such temperatures and also because of problems associated with water management in the cells when operating below 0° C. A certain amount of water is required for proper fuel cell operation (e.g. for hydration of the membrane electrolyte) and water is also generated as a result of providing electrical power. However, ice of course forms where liquid water is present at such temperatures. The presence of ice can be problematic depending on how much there is and its location when storing or when starting up a fuel cell stack. The formation of ice in the electrochemically active MEAs of the fuel cells is particularly problematic during startup from below freezing temperatures.
As a result of the importance of this issue and the difficulties involved, numerous fuel cell designs and start-up methods have been proposed in the art to address the various problems encountered during start-up from temperatures below freezing. In addition though, various methods have been proposed for appropriately shutting down and storing fuel cells in anticipation of below freezing storage conditions. For instance, a method has been proposed in US20070298289 which involves determining the potential that a freeze condition will exist after the system is shut-down based on predetermined input, such as ambient temperature, geographical location, user usage profile, date, weather reports, etc. If the system determines that a freeze condition is probable, then the system initiates a purge shut-down of the fuel cell system where water is purged out of the reactant gas flow channels. If the system determines that a freeze condition is unlikely, then it will initiate a normal shut-down procedure without purging the flow channels. The system will then periodically determine if the conditions have changed, and can initiate a “keep warm” strategy if it is determined that a potential freeze condition exists. If the fuel level is insufficiently high to keep it warm however, the system will instead initiate the purge.
In another example, US20140093801 discloses a system and method for selectively determining whether a freeze purge should be performed at shut-down of a fuel cell stack. The method includes identifying that the vehicle has been keyed off and then determining whether a stack membrane humidification value (lambda) is less than a predetermined humidification value that identifies the humidification of membranes in fuel cells in the fuel cell stack. If the stack membrane humidification value is not less than the predetermined humidification value, then the method determines if the ambient temperature is below a predetermined ambient temperature, and if so, performs the freeze purge. If the ambient temperature is not below the predetermined ambient temperature, then the method performs a short non-freeze purge of the flow channels in the fuel cell stack. The method determines a wake-up time for a controller for a next time to determine whether a freeze purge should be performed.
Despite the advances made to date, there remains a need for improved methods for appropriate shutdown and storage of fuel cell systems in subzero temperature conditions. This invention represents an option for fulfilling these needs and provides further related advantages.